This invention relates generally to bandgap reference voltage generators and more particularly, it relates to an ECL bandgap reference voltage generator whose operation is compensated to produce an output reference voltage V.sub.BB that is independent of variations in supply voltage.
As is generally well-known, emitter-coupled logic (ECL) is a widely utilized logic family for high performance products due to its very short propagation delay. Therefore, in order to preserve the high performance of integrated circuits embodying ECL logic, a bandgap reference voltage V.sub.CS has been commonly generated heretofore on-chip and is used to control the base of the main current source transistor. This reference voltage V.sub.CS has the characteristic of being stable and it tracks variations in processing and changes in operating parameters such as temperature. Such a prior art bandgap reference generator 2 for producing a reference voltage V.sub.CS is illustrated in FIG. 1 and has been labeled "Prior Art." However, the reference voltage V.sub.CS suffers from the disadvantage of still varying over the supply voltage VCC.
In order to eliminate the dependence on variations in the supply voltage, a "VCC Compensation" circuit 4 was developed in the prior art and added to the bandgap reference generator 2. In operation, the transistors Q8, Q7 and Q6 in the "VCC Compensation" circuit 4 function similar to the operation of the transistors Q1, Q2 and Q3 in the bandgap reference generator 2 so that the collector current flowing through the transistor Q6 would have a specific relationship to the collector current flowing through the transistor Q3. The supply voltage VEE is generated externally and is provided to a packaged integrated circuit through a dedicated pin.
For ECL circuits, the upper supply voltage VCC is 0 volts and the lower supply voltage VEE is typically -5.0 volts, but is still considered to be acceptable if it lies within a range of .+-.10%. Thus, at a high lower supply voltage (VEE=-5.5 volts), the current through the current source resistor R9 is increased. In order to maintain a constant current to the collector of the transistor Q3, the current through the collector of the transistor Q6 must be increased. At a low lower supply voltage (VEE=-4.5 volts), the current through the current source resistor R9 is decreased. Therefore, in order to maintain again a constant current through the collector of the transistor Q3, the current through the collector of the transistor Q6 must be decreased. As a result, the current flowing through the collector of the transistor Q3 will be independent of variations in the supply voltage VCC.
It will be noted that the "VCC Compensation" circuit 4 is connected to the collector of the transistor Q3 and thus provides for additional circuit paths between the supply voltage VCC and the ground potential. As a consequence, the "VCC Compensation" circuit 4 has the undesirable feature of consuming additional power, which may in fact be more than the power consumption of the bandgap reference generator 2 itself.
In ECL circuits, a stable reference voltage V.sub.BB is also generated from a bandgap reference voltage generator and is supplied to the base of a reference transistor in order to establish the threshold level for the recognition of a high or low logic state. It would therefore be desirable to provide an improved bandgap reference generator, which includes compensation circuitry for supplying a reference voltage V.sub.BB that is independent of variations in supply voltage, but yet consumes no additional power. This is achieved in the present invention by the provision of compensation circuitry comprised of a supply-independent current source which supplies a constant current to the collector of a constant current source transistor in a bandgap circuit portion.